1. Field of the Invention
The present invention generally relates to high resolution linear or rotary encoders used for determination of position and/or time derivatives of position. The present invention particularly relates to optical encoders of position.
2. Description of the Relevant Art
Encoders of position transform a physical position into an electrical signal corresponding to such position. Encoders of position which are optical employ a source of light, a detector of light from such source, and a grating which positionally moves relative to the path of light between the source and the detector in order, by such movement, to selectively interrupt the light path between the source and the detector. The optical path may be either through the grating or reflected from the grating. The position being encoded may either be angular (rotary) or linear. The position may be simultaneously encoded on either single or multiple channels of encoding. The absolute position relative to some predetermined reference position may be encoded, or only the incremental positional changes may be encoded. Most positional encoding is, however, of the incremental type wherein an alternating electrical signal is produced with successive incremental positional movements, each detectable increment of position producing one phase change of the electrical signal.
The general fields of application for positional encoders, and particularly for optical position encoders, are diverse. They include inspections, measurements, and metrology; factory automation including machine tools, robotics, and assembly machinery; semiconductor processing equipment; medical therapuetic and diagnostic equipments; and computer peripherals including printers and disk drives. Specific applications include Coordinate Measuring Machines (CMM), Flexible Machining Centers (FMC), X-Y stages, Servo systems, rotary indexing tables, component insertion machines, photomask inspection machines, medical ultrasound systems, tape drives, printers, plotters, microfiche readers, and phototypesetters.
An incremental optical positional encoder is simple in concept. It normally consists of four components. First there is a source of light, which source is normally either incandescent or a Light Emitting Diode (LED). In the optical transmission path is an assembly, normally an encoder strip for a linear encoder or an encoder wheel for a rotary optical encoder, which presents a pattern of alternating opaque and translucent segments. The light passing through (or reflected from) such segments is alternately transmitted and blocked (or, alternatively, reflected or absorbed) causing a light sensor, which is usually a phototransistor, to detect a light and dark modulation in the light beam. Finally, electrical signal conditioning circuitry is used to format the electrical signal generated by the light sensor (phototransistor) into usable information. The beam from a small light source, normally an L.E.D., which is collimated in an elementary optical encoder of this nature will normally have a positional resolution on the order of 10+ lines per inch.
Attempts to extend this elementary concept of optical position encoders to high positional resolution levels within reasonably sized packages rapidly result in considerably increased design sophistication, and greatly increased cost on the order of hundreds of dollars per each high resolution optical encoder assembly. Particularly in order to obtain increased resolution in an optical position encoder (i) the light source is collimated and (ii) an additional element called a mask, or reticle, is added between the optical disk and the sensor. This mask, or reticle, produces a shuttering effect so that only when the translucent segments of both the encoder disk and the reticle are in alignment is light then permitted to pass in the path between the light source and the light sensor. Optical encoders employing both collimated optical beams and masks, or reticles, can typically resolve on the order of 144+ lines per inch.
This improved level of resolution, and even that somewhat greater level of resolution attained by the very best prior art optical encoders which are of reasonable size (and which will shortly be specifically discussed), is very often inadequate in modern applications. For example, an important specification of robot performance and productivity is its acceleration/deceleration times. However, there is no good measurement standard to mathematically describe overshoot and settling times resultant from acceleration and deceleration of a robot arm. Despite the lack of quantification, the robot movement process is clearly visible. When a robot arm approaches a designated point that requires maximun repeatability under full payload, it is all too likely to exhibit an overshoot or settling time problem which causes it to do a "rain dance", jerking every which way. Another robot specification which correlates with the existence of these problems is encoder shaft pulses per second, or resolution. The more pulses per second generated by the encoder shaft at full speed, the more precise the possible control of the robot arm and the less likelihood of overshoot and/or settling time problems which, in the extreme case, result in crashes and damaged parts.
Even when the positional encoder rate in pulses per second is very high, or even maximal, by the standards of the prior art, it should be recognized that electronic circuits which are carefully designed to apply time constants reflective of mechanical settling times are required to effect even approximately smooth movement. This is true even though the electronic circuits controlling mechanical motion may be many hundreds, and even thousands, of times faster-responding than the mechanical systems which are controlled in motion. Why then cannot electronic control of position be simplistically based on mere brute force positional feedback loop computation and resultant control?. The answer is simply that, in the past, the electronics needed to be sophisticated to effect control based on inadequately precise and inadequately timely positional information. The electronics must constantly extrapolate from imprecise and untimely information to predict where the mechanical system really is, and will be. It is generally insufficiently accurate and/or timely positional information, and not any intrinsic limitation of electronic control of mechanical motion, which imparts the jerkiness to robot motion which is so cleverly and amusingly satirized in a human pantomine of such robot motion.
The present invention is intended, amongst other purposes, to render obsolete the stereotypical notion of jerky and spasmodic robot motion. It does so by providing low cost positional sensing of such high resolution that even quite crude electronic positional control circuits will suffice, in combination with improved positional encoders in accordance with the present invention, to so continuously apply accurate drive stimulus to mechanical motion so as to make such motion appear, in relation to the human senses, to be fluid and graceful. In other words, if position, and the first time derivative of position or velocity, and the second time derivative of position or acceleration, may all be readily known with astounding accuracy and time currency, then the most rudimentary equations relating actual future position to desired future position may be used to effect control of the motion of mechanical systems. Before positional encoders in accordance with the present invention are taught, however, it is useful to further consider specific prior art optical encoders of high performance.
Representative highest-performing prior art optical rotary position encoders include the following. The Encoder Division of Dynamics Research Corporation offers in their Module 25 encoder a disk which can be provided with up to 3,000 lines, providing a maximun of 3,000 cycles per shaft revolution (exclusive of either internal or external cycle interpolation) in a 2.5 inch diameter package. The K3 series modular optical encoder from the Instrument Division of Dresser Industries is capable of up to 2500 cycles per revolution resolution in a 2.1 inch diameter case. Finally, the HEDS-6000 series of incremental optical encoders from Hewlett Packard offers resolution of up to 1024 cycles per revolution in a 2.2 inch diameter case. The number of lines, or transitions, which are being resolved per inch in these encoders may be readily estimated from the fact that the circumference of a circle is pi times its diameter.
All the prior art high resolution high optical encoders are very difficult and exacting of assembly, which contributes greatly to their cost. For example, the aforementioned HEDS-6000 series is available as a user-assemblable optical encoder kit. The number of major assembly steps is 8, each consisting of an average of 4 seperate substeps. Although in high volume applications using custom design tooling and automated equipments it is predicted by the manufacturer that encoder assembly can be accomplished in less than 30 seconds, assembly by more conventional manual means is a demanding task of many minutes which is normally performed by skilled technicians or assemblers.
The difficulties, time demands, and resultant high cost of assembling high-resolution optical encoder assemblies stems from their basic and uneliminatable requirement for exacting mechanical and optical parts which must be aligned with great precision. Consider, for example, the sensitivity of a rotary optical encoder to motor shaft runout and woble. Motor shaft runout means that the optical encoder disk may be eccentric on its axis of rotation. This means that the alternating opaque and translucent segments, as detected at the radius of the encoder disk whereat it intercepts the light path, will not be of equal width in different sectors around the circumference of the optical disk. Unequal widths translate into unequal detected light energy, and unequal electrical signals resultant from such detection, for equal motion. Obviously positional resolution, and accuracy, is affected when the transition of the electrical signal may be a function of which sector of an eccentric encoder disk is being read as well as the motion and position of the encoder disk.
Probably more important than motor shaft runout, which to some degree can be overcome with electrical signal shaping circuitry, is the problem of wobble, or that the plane of the disk may not be precisely perpendicular in all sectors thereof to the path of light through such encoded disk. If the alternating opaque and translucent segments on the disk are at a slight angle to the path of light intercepting such segments, than it should be envisioned that these segments, being of finite thickness, will intercept the light beam in a manner which causes more of a step function, or even a sine wave, in the light intensity detected at the light sensor as opposed to an on-off square wave of received light intensity. Thus, when the alternating segments, which are extremely narrow, are at a sufficient angle to the impingent light beam, then no modulation will be obtained at all, with the leading edge of one opaque segment overlapping the trailing edge of a predecessor opaque segment. In order to reduce this problem, the optical encoder disk may be made extremely thin. If it is thin and flexible then it may exhibit warp or systemic deformation, reintroducing wobble. If it is extremely thin and rigid then it may be readily subject to mechanical damage, especially from shock.
Problems experienced with the demanding mechanical components, and alignment, of high resolution optical positional encoders are analogous to those experienced in phonographic reproduction of sound, in magnetic recording of disks, and in optical recording of disks. In order to obtain resolution performance at or near the limits of the prior art technology optical encoders, which limits are on the order of several hundred lines per inch resolution, cosiderable penalties may have to be paid in reliability, immunity to vibration and/or mechanical shock and/or temperature variation, and especially in cost. For these reasons, the present invention is embodied in a new apparatus for highest resolution optical position encoding which apparatus is both easy and low cost of assembly, and rugged and reliable in operation.